Process control plants or systems often employ fluid valves, such as rotary valves, linear valves, etc., to control the flow of process fluids. In general, fluid valves typically include a fluid flow control member that is disposed in the fluid path and which is operatively coupled to an actuator such as, for example, a pneumatic actuator, a manual actuator, etc. A valve stem or shaft operatively couples the flow control member to the actuator, which moves the flow control member between an open position and a closed position to allow or restrict fluid flow between an inlet and an outlet of the valve. Typically, the valve stem or shaft extends between the flow control member and the actuator through a bonnet coupled to the valve body.
A valve packing or packing assembly is often employed to prevent undesired leakage of process fluid to the environment through the valve body along the valve stem or shaft. The valve packing may include a plurality of springs and/or packing seals disposed along a portion of the valve stem or shaft to provide a seal. Some known valve packings employ a high-spring rate loading device or spring assembly (e.g., Belleville springs) to provide a relatively high load over a relatively small range of deflection or compression. However, such known loading devices typically require very precise deflections and/or tightly held manufacturing tolerances to enable a desired packing stress to be applied to the packing seals.
Failure to provide a desired packing stress to the packing seals may cause an improper seal. For example, a packing stress that is too low may cause the process fluid to leak to the environment through the packing seals. A packing stress that is too large (e.g., greater than a maximum recommended packing stress) may cause some types of packing seals (e.g., graphite seals) to transfer material to a valve stem, thereby causing build-up of material on the valve stem and damaging the packing seal. Additionally or alternatively, packing stresses that are too high may increase packing friction between the packing seals and the valve stem or shaft, which may reduce valve performance and/or the operational life of the valve packing and/or the valve stem or shaft.